Ransher:
i recently came know it's bcoz of overflowing the Serial3 software receive buffer and i don't the solution.
Every 100 milliseconds you are sending 12 or 13 characters to Serial3 and only reading 1 back.
Try:
void loop()
{
Serial3.println("hello world");
while (Serial3.available())
Serial.write(Serial3.read());
delay(100);
}
That should pull characters from the input buffer fast enough that the buffer doesn't overflow.